This invention relates to a method of electrowinning metals and to cells and anodes for use in connection with such a method.
The extraction and recovery of metals from acqueous acidic solutions by electrowinning involves the use of an insoluble anode in an electrolytic cell. The aqueous solution forms the electrolyte, and a suitable cathode, usually in the form of a plate or sheet on which the metal is deposited, forms the remaining part of the cell. Under suitable conditions, metal of very high purity can be deposited, and the process is used for winning metal from solutions derived from metal ores, scrap metal and from metal refining processes. In particular, copper, nickel, cobalt, manganese and zinc are common metals which may be extracted from acid solutions by such a process and such solutions are completely or partly stripped of their metal content without significant replenishment from the anode material. Electrowinning is to be distinguished from electrorefining in which the anode is a soluble anode principally composed of the metal which is to be deposited on the cathode.
Ideally, the anode used in electrowinning should be completely resistant to the electrolytic conditions, should suffer no weight loss, should not form a passive film or give rise to any reaction which would interfere with the cathodic deposition of metal at an economic current density. In practice, these conditions cannot be fulfilled by any economically realistic anode and all of the anodes presently being used have a small amount of dissolution which may affect the purity of the metal deposit. Thus, lead, which is widely used in electrowinning, is not completely resistant to anodic loss and some lead becomes co-deposited with the copper, thereby reducing the purity and hence the commercial value of the copper deposit. Whilst the wear rate of lead may be acceptably low for low current density electrolytic cells, some lead is picked up in the cathode and the wear rate increases in certain commonly found conditions. These are increasing current density, increasing acidity, increasing electrode temperature, and increasing chloride concentration. With the increase in current density, lead tends to oxidise and flake away from the anode and this increases the removal rate to an unacceptable degree. Lead in copper has a deleterious effect on the conductivity of the copper and it also adversely affects the ability to draw the copper to fine wire.
Another form of non-consumable anode is titanium,, either commercially pure or alloyed, which is coated with some form of noble metal or noble metal compound. Titanium on its own is unsatisfactory because it rapidly polarises when made anodic. By polarisation is meant the rapid formation of an insulating film, usually an oxide, on the exterior surface of the anode material such that it is unable to pass a current at a voltage which is economically viable in a commercial electrowinning cell. Whilst noble metal coated anodes are technically attractive for electrowinning operations, there is a very small loss of noble metal, which although not significantly affecting the purity of the deposited metal, does adversely affect the economics of the process because of the very high cost of the noble metal wastage and the high capital costs of the electrode.
In the electrorefining of copper, insoluble anodes have been proposed which consist of alloys of copper and silicon with additions of iron, tin, lead and manganese. These alloys, available commercially under the trade name "Chilex", have been used in Chile and are cast into suitable shapes for use in the electrolytic bath. These alloys have been in use for 40-50 years. The wear rate of these alloys is about 10-25mm/year depending on electrolyte conditions and the current density. The wear is principally due to copper passing into solution, which copper is then eventually deposited on the cathode. This dissolution is not in itself objectionable since it does not contaminate the cathode deposit, but it does mean that the anode has to be replaced periodically.
In the electrowinning of cobalt, insoluble anodes of cobalt-silicon have been proposed, these alloys being commercially available under the trade name "Luilu". These "Luilu" alloys are again cast into suitable anode shapes and wear gradually in use. Since they are used in cobalt electrolysis, the cobalt is recovered from the anode and is not lost. Both copper-silicon alloys and cobalt-silicon alloys are relatively brittle and can become damaged during transit and handling which again reduces their effective life and increases their costs.